One of the main areas of use of the laser pump source is in the optical pumping of fibre amplifiers for use in optical telecommunications systems.
Common optical amplifiers include rare earth doped fibre amplifier such as Erbium doped amplifiers. Such amplifiers demonstrate high gain and low noise. These devices also operate to amplify signals in the wavelength range 1.5 to 1.6 microns and as such cover the 1.54 microns window in silica optical fibres used in telecommunications.
Such amplifiers require pumping in one of their absorption bands at the 807 nm, 918 nm or 1480 nm. Multi-mode pumping may be used to pump fibre amplifiers using a method known as cladding pumping in which specially designed Erbium doped fibres are pumped by a multi-mode pump laser via the cladding of the Erbium doped fibre. However, this is a complex process requiring a special fibre design and is not suitable for most amplifiers. Also, this system can not be retrofitted to pump presently deployed amplifiers that do not include such fibres.
Single mode pumping may also be used. In this case a relatively low power single mode pump may be used to pump a single Erbium fibre amplifier or the output from a higher power, normally 980 manometer, single mode pump laser may be split to pump a plurality of fibres. In this case, passive fibre splitting would be used to provide the plurality of outputs. This method has its drawbacks in that single mode pumps are expensive in comparison to multi-mode pumps and the reliability of high power 980 manometer single mode pumps decreases super linearly (approximately cubicly) with output power. Hence, there will be reliability issues if a high power single mode pump is used to pump a plurality of Erbium doped fibre amplifiers.